This disclosure relates generally to patient monitoring. More particularly, the present invention relates to generation of alarms in physiological monitoring apparatuses, termed patient monitors below.
Patient monitors are electronic devices designed to display physiological information about a subject. Electrocardiogram (ECG), electroencephalogram (EEG), plethysmographic signals, and signals related to blood pressure, temperature, and respiration represent typical physiological information contained in full-size patient monitors. Patient monitors are typically also furnished with alarming functionality to alert the nursing staff when a vital sign or physiological parameter of a patient exceeds or drops below a preset limit. Alarms are normally both audible and visual effects aiming to alert the staff to a life-threatening condition or to another event considered vital.
In addition to individual sensor/parameter alarms, patient monitors may be configured to raise combinatory alarms. That is, several physiological parameters may be used to determine a combined index and to give an alarm when the combined index fulfills a specific criterion. The combinatory alarms may range from simple combinations like “low heart rate and low arterial pressure” to complex rule-based scenarios used in various clinical support systems, for example. Below, the term physiological parameter is used to refer to the physiological variable to be monitored. As discussed above, the variable may be an individual parameter, such as heart rate or blood pressure, or a combinatory variable/index derived from multiple individual parameters. An individual physiological parameter may also represent a waveform signal value determined over a predefined period of time.
In most monitors, the alarm limits of a physiological parameter may be defined by the user, since the limits typically depend on patient etiology, age, gender, medication, and various other subjective factors. Each physiological parameter may also be assigned more than one alarm limit/criterion. That is, for a specific physiological parameter a patient monitor may raise alarms of different levels.
Alarm generation is a demanding task, as the patient monitor should be both sensitive and specific in producing alarms. In other words, the monitor should be able to recognize all true alarm events, without raising false or clinically irrelevant alarms. The difficulty of this task reflects in a real clinical environment where a large fraction of the alarms, even most alarms, may be considered to be false or at least clinically irrelevant. Such a large number of false or irrelevant alarms causes an enormous burden on the nursing staff and may also lead to impairment of the responses to true alarms.
It has been suggested to wait a given time period after the parameter crosses an alarm limit, thereby to reduce alarms caused by very short-time crossings that are likely to be caused by noise or signal artifacts. It has also been suggested to reduce nuisance alarms in a pulse oximeter by determining both the amount of time the measured value is past the limit and the amount by the limit is passed, and to inhibit an alarm based upon a combination of the said two amounts. The combination may be an integral that represents the area formed between the parameter envelope and the alarm limit after the crossing of the limit.
A drawback related to the alarm generation methods that start to analyze the signal and the nature of the alarm limit crossing in response to the detected crossing is the potential latency related to alarm initiation and also to alarm termination, especially if the parameter fluctuates near the alarm limit for a longer time and then a more sudden change occurs for the worse. This may be the case, for example, for a so-called decompensating patient whose body functions first try to compensate for the worsening physiological state. Consequently, the information obtained from the parameter is too positive during the compensation and the subsequent alarm may be delayed and assigned a low priority.